Generally accepted practice for reducing friction when two surfaces move relative to one another is to employ greases and oils as lubricants. Such lubrication, known as hydrodynamic lubrication, results in very low coefficients of friction, of well below μ=0.05. Since, under the surface contact stress that occurs, the grease is displaced over time from the area of contact between the two surfaces, and/or may undergo resinification as a result of contamination and oxidation processes, the surfaces in question must be regularly regreased in order to maintain the lubricating effect. This entails increased maintenance effort and an additional cost factor in the case of plants, and also, possibly, component failures in cases where maintenance is not regular. Coatings which diminish sliding friction, consisting of a polymeric binder and solid-state lubricants, may be used as an alternative to this. Significant disadvantages, however, are the generally relatively high coefficients of friction, at levels of μ=0.1-0.2, and the need to balance minimal sliding friction with a controllable abrasion behavior of the layers in association with the formation of the transfer film that is important for lubrication. In the case of corroding substrates, moreover, sufficient corrosion protection must be provided, since the absence of grease means that the hydrophobic protective function in the overall system is lost, and hence electrolytes and oxygen as well may have easy access to the substrate surface. Introduction of abrasion resistance and corrosion protection is accomplished, as a general rule, by using dispersion to incorporate suitable inorganic particles, which in their forms as presently available do fulfill their protective function, but additionally exert a deleterious influence over the lubricating effect of the layer system, so making the resultant compositions of no interest for practical application in the majority of cases.
U.S. Pat. No. 4,694,038 A (Boeing) claims a coating that reduces sliding friction and consists of a polyurethane binder and 20-40 wt % of amorphous graphite particles suitable for equipping the surfaces of elastomers. Using a polyurethane binder allows for effective adhesion to the substrate. A disadvantage, however, is that the graphite particles are distributed uniformly over the thickness of the coating material. Accordingly, while the desired low sliding friction to the outside of the coating is achieved, owing to the capacity for the graphite to be eroded in layers, but at the same time the presence of the graphite in the vicinity of the boundary layer with the substrate causes reduction in the layer adhesion to the substrate material, which can be controlled to some extent only by using correspondingly reactive binders (polyurethanes). Furthermore, as a result of the high level of solid lubricant, the layer systems become relatively soft, and under high surface contact pressures this may easily lead to layer failures as a result of microplowing.
With a similar principle of the uniform distribution of particles over the binder matrix, U.S. Pat. No. 5,789,523 A (DuPont) claims a polyimide composition with incorporated soft phyllosilicates (Mohs hardness 1-5) and carbon fibers, and also, optionally, solid-state lubricants featuring improved sliding friction and improved abrasion resistance, for the purpose of producing injection moldings with built-in tribological effect. The phyllosilicates are said to enhance the durability of the composite materials. At the same time, through the equal distribution of the inorganic adjuvants over the organic polyimide matrix, and through the simultaneously required high levels of adjuvant filling for the purpose of achieving suitable tribological surface properties, there is likewise significant influence over the mechanical and thermal bulk properties of the molding produced, such as the elasticity modulus or the coefficient of thermal expansion, for example, and in certain applications this may be entirely unwanted.
In contrast to the aforementioned references for which the platelet-shaped particles are dispersed only mechanically using shearing forces, WO 2002005293 A2 (EMTEC Magnetic) claims an electrically conductive layer of graphite and a binder, to which a polyurethane dispersion resin is added that carries at least one polar functional group along the main polymer chain that is able to interact with the surface of the platelet-shaped graphite filler and is also in a position to form corresponding graphite intercalation compounds. This type of modification likewise results in equal distribution of the particles over the binder matrix, with a finer particle size distribution, as a general rule, than in the case of purely mechanical dispersing using shearing forces. A morphology of this kind is also desirable in order to achieve high electrical conductivity. Tribological properties, however, are not claimed.
Low-friction coatings of these kinds, comprising solid-state lubricants and polymeric binders, are also frequently equipped, for mechanical reinforcement, with particles of hard material (US 20040229759 A1 (Jet-Lube Inc.)). This principle functions to start with via a simple mixing rule. A problem, however, is that the hard material particles at relatively high concentration generally exert an abrasive effect on the tribological system, as they are transferred onto the opposing body when a transfer film is formed, and may lead to greater wear in the tribological layer in the subsequent friction process. U.S. Pat. No. 4,898,905 A (Taiho Kogyo) likewise claims a lubricant varnish composition comprising polyimide matrix, platelet-shaped, solid-state lubricants, platelet-shaped, silicatic additives, and oil. In this case as well, no particular measure is taken in order to generate a controlled arrangement of particles over the binder matrix, and nor is any such arrangement obtained. The oil that is used introduces into the system a hydrodynamic lubricant component, and by this means, in particular, the initial frictional behavior and the level of the coefficient of sliding friction can be influenced. The fact that the oil may diffuse in the layer and is removed from the system over time is a disadvantage. As a result, the effect is not long-lasting.
U.S. Pat. No. 3,809,442 (3M), EP 1350817 A1 (Ford Motor Comp.), and WO 2005010107 A1 (TNO) claim low-friction coatings comprising combinations of various solid-state lubricants with binders, which are designed for low-temperature applications and coating of temperature-sensitive substrates. WO 2005010107 A1 additionally requires a further, polysiloxane-based, polyolefin wax-based and/or PTFE-based lubricating additive. To develop its activity the additive must not interact with the solid-state lubricants and must be able to diffuse unhindered to the layer/air phase boundary. Any special morphological disposition of the particulate constituents is not critical to the activity of the coating. The additive leads merely to a hydrodynamic component in relation to the lubrication, similar to that described above.
Polymer matrix composites with reinforcing particles are also described for abrasion-resistant antistick coatings.
EP 1718690 claims abrasion-resistant, low-energy layers featuring enhanced alkali resistance. The physical composition comprises a curable organic binder system, at least one functionalized, fluorine-containing polymer or oligomer that is reactive with the binder, and inorganic particles as well.
Owing to the abrasive nature of the particles of hard material, tribological properties cannot be inferred and are also not claimed. Nor is there any description of inorganic, solid-state lubricants.
As a result of the present invention it has been possible to provide a tribologically active layer system which by virtue of its special structure compensates the above-stated disadvantages of conventional low-friction coatings.
It was an object of the present invention to provide a pigmented, fine-structured, tribological composite material which combines a low coefficient of sliding friction with excellent substrate adhesion and with outstanding abrasion and wear resistance, in association with a high barrier function with respect to the diffusion of water vapor, gases, and also corrosive media.